This invention relates to tomographic imaging systems, such as x-ray computed tomography. More specifically, the invention relates to a method for reducing image artifacts caused by over-ranged or "clipped" signals acquired by such imaging systems.
Tomographic imaging systems, as considered herein, are imaging systems which produce a tomographic or "slice" picture of an object such as the human body. Such tomographic systems collect a series of "projections" at various angles around the body, each projection made up of measurements of radiation emitted from the body. The radiation may be that transmitted through the body by an external radiation source, such as an x-ray tube, as in X-ray computed tomography ("CT") or the radiation may be that emitted from the body from internally placed radio-pharmaceutical isotopes, e.g. T.sub.c.sup.99 used in Single Photon Emission Computed Tomography (SPECT).
In an x-ray CT machine, an x-ray source is collimated to form a planar x-ray beam within an x-y plane of a Cartesian coordinate system. The beam is transmitted through the imaged body and received by a generally linear detector array also within this x-y plane.
The detector array is comprised of a plurality of adjacent detector elements each receiving radiation along a single "ray" from the focal spot of the x-ray source to that detector element. Each detector element produces a signal indicating the attenuation of the x-ray beam along that ray by the imaged body. The detector elements, the signal from each detector element, and the ray of the fan beam are all generally referred to as a "channel" of the projection, the particular reference being clear from context.
In one common embodiment, the planar x-ray beam is a "fan beam" radiating from a focal point and the detector array has its elements organized in an arc of constant radius about the focal spot. The x-ray source and detector array may be rotated on a gantry around the imaged object so that the fan beam intersects the imaged object at different angles.
A number of projections are acquired at different gantry angles to form a tomographic projection set. The acquired tomographic set is typically stored in numerical form and may be "reconstructed" by mathematical techniques to yield a slice image. The reconstructed images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The natural evolution of x-ray CT has led to the development of higher powered x-ray sources. Such x-ray sources produce increased x-ray flux which is desirable for two reasons: First, increased flux improves the signal-to-noise ratio in the resulting image, for example, by minimizing the effect of noise. Second, increased flux permits the acquisition of the projection set during a shortened scanning time. Decreasing the scanning time improves patient comfort and helps minimize image artifacts caused by patient motion.
The use of increased x-ray power in an x-ray CT system creates the potential of overwhelming the system's detector signal chain. In particular, the analog-to-digital converter ("ADC") associated with the CT system's data acquisition system ("DAS") may be driven over its range. Such an over-range condition artificially limits the signal from the over range channels to the maximum value of the ADC and causes the data of these channels to be effectively lost.
In the case of a patient scanned by a CT system, the over-range channels will typically be those channels which receive the peripheral rays of the fan beam, e.g. those near the outer edges of the patient. Although these areas of the patient may be diagnostically less important, errors in the data from any channel will cause artifacts in the entire reconstructed image. This spreading of the effects of locally erroneous projection data is caused by the frequency domain filtering implicit in the image reconstruction process, such filtering which spreads the effects of erroneous data over the image.
The problem of over-range channels is similar to the problem created by the truncation of projections when the imaged object extends outside of the fan beam and detector range. Such truncation may occur, for example, when the patient's arms are outside of the fan beam for some projections and in the fan beam for other projections.
U.S. Pat. No. 4,550,371, incorporated by reference and assigned to the same assignee as the present application, provides a means for correcting a truncated projection by calculating the moments of all projections. Unfortunately, this method relies on the assumption that the majority of the projections are not truncated, an assumption which does not hold, in general, for projections having over-range channels.
One proposed method of correcting for the effect of over-range channels involves independently measuring the thickness of the imaged object along the rays associated with the over-range channels and substituting an attenuation value for those rays based on assumption of constant density of the object along those rays. Although some success has been achieved with this approach, the requirement that the thickness of the body be measured independently is cumbersome and commercially impractical.
Ideally, any correction system should operate quickly enough so as not to significantly delay the production of tomographic images from the projection set. Preferably, the correction system should allow correction to begin as the projections are being acquired and should be susceptible to parallel processing in an array processor or the like, such processors as are commonly used for the reconstruction of tomographic images.